While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, DUV and VUV lithography is thought to hold particular promise as the next generation in microfabrication technology. At present, the manufacture of advanced semiconductor devices having a feature size of 0.15 μm is conducted by photolithography using a KrF excimer laser, and even 0.13-μm rule devices are on the verge of commercial manufacture. It is strongly desired that photolithography using an ArF excimer laser as the light source reach the practical level as the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
In the photolithography using an ArF excimer laser (wavelength 193 nm) as the light source, a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing a high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, poly(meth)acrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene and metathesis ring-opening polymers have been proposed as the base resin. This choice is effective in that the transparency of a resin alone is increased. However, the photoacid generator has the problem that increasing its transparency leads to a drop of acid generation efficiency, resulting in a low sensitivity or the lack of thermal stability and storage stability. There is available no photoacid generator which is practically acceptable.
For example, JP-A 7-25846, JP-A 7-28237, JP-A 8-27102, JP-A 2001-354669, and JP-A 2002-40636 disclose alkylsulfonium salts which are highly transparent, but unsatisfactory in acid generation efficiency and thermal stability. The following salts are illustrated in JP-A 7-25846, JP-A 2001-354669, and JP-A 2002-40636.

JP-A 10-319581 discloses alkylarylsulfonium salts which have a high sensitivity and a good balance of transparency and acid generation efficiency, but lack thermal stability and storage stability.

Further, JP-A 8-146607, JP-A 9-118663, JP-A 10-48814, JP-A 10-232490 and JP-A 2002-229192 disclose sulfonium salts having naphthyl groups, and describe that resist compositions comprising the same exhibit a high sensitivity, high resolution and improved pattern configuration. These naphthyl group-containing sulfonium salts have a high transmittance near 193 nm, but an inferior sensitivity to the conventional triarylsulfonium salts. The sensitivity can be enhanced by increasing the amount of the salt added to resist compositions, but noticeable demerits are introduced. That is, adding large amounts of low-molecular weight components to resist compositions can cause degradation of dissolution properties, precipitation of substantially insoluble sulfonium salts, and conversion thereof into foreign matter.
Additional drawbacks are that for example, trinaphthylsulfonium salts and 2-dialkylnaphthylsulfonium salts are difficult to produce, and that 1-naphthyldialkylsulfonium salts which are unsubstituted (hydrogen atom) at 2-position lack storage stability in resist solution.
The following salts are disclosed in JP-A 8-146607, JP-A 9-118663, JP-A 10-48814, JP-A 10-232490 and JP-A 2002-229192.

Arylsulfonium salts, which are regarded effective in photolithography using a KrF excimer laser, are good in acid generation efficiency, thermal stability and storage stability, but very low transparent to light so that the pattern resulting from exposure and development is noticeably tapered. The lack of transparency can be compensated for by thinning the resist film, but such a thin resist film has extremely low etch resistance. This is inadequate as the pattern forming process. Most of the foregoing salts are onium salts whose cation side structure is modified. It was reported that with respect to resolution and pattern configuration, there is a close relationship between the type of acid generated and the type of acid labile group.
With the advance toward a finer feature size, line edge roughness and a size difference between an isolated pattern and a densely packed pattern, known as I/G bias are regarded problematic. It is well known that even when feature sizes are the same on the mask, a size difference appears between an isolated pattern and a densely packed pattern after development. This problem becomes serious with sizes in excess of the wavelength. This is because a difference in light interference upon image formation between an isolated pattern and a densely packed pattern brings about a difference in optical intensity. The resist size decreases with an increase of pitch (pitch=the sum of line size and space size, in this case, the line size remains unchanged and the space size is increased) and becomes increasingly thinner with the enhancement of acid diffusion. The problem of size dependency on line density that the size of an isolated pattern is thinner than that of a densely packed pattern becomes serious. One proposed approach for reducing the line density dependency is to reduce the distance of acid diffusion. However, if acid diffusion is extremely restrained, the side walls of resist patterns after development are serrated or roughened by standing waves, and line edge roughness is enhanced. It has been reported that the serration of side walls by standing waves becomes sharper as the distance of acid diffusion is reduced. For the line edge roughness as observed under top-down SEM, the same tendency is ascertained, that is, line edge roughness increases as acid diffusion decreases. A common approach for reducing the roughness of lines is by increasing the distance of acid diffusion, but this approach fails to improve the line density dependency over a certain limit. For improving line edge roughness, it may be effective to increase the optical contrast. For example, at the same exposure wavelength, the line edge roughness decreases as the line width increases. Even at the same exposure wavelength and line width, the line edge roughness decreases with an increasing NA of a stepper and in the case of repetitive patterns, is smaller with modified illumination (e.g., annular illumination, quadrupole illumination) than with normal illumination and with phase shift masks than with conventional Cr masks. The contrast at pattern line edges is correlated to the line edge roughness so that the line edge roughness becomes smaller as the line edge contrast becomes sharper. With respect to exposure wavelength, it is expected that the line edge roughness becomes smaller upon exposure at shorter wavelengths. However, when line edge roughness is compared between KrF exposure and ArF exposure, the ArF exposure is deemed to provide a higher optical contrast owing to its shortness of wavelength and a smaller line edge roughness, but actually, the KrF exposure is advantageous, as reported in SPIE 3999, 264 (2000). This is attributable to the performance difference between KrF and ArF resist materials, indicating that the line edge roughness originating from material factors upon ArF exposure is serious. It would be desirable to have a photoacid generator which improves line edge roughness and at the same time, does not exacerbate line density dependency.